This invention relates to photographic optical systems with photoelectric focus detectors, and more particularly, to photographic optical systems in which the decrease in either the amount of projection of a beam for measuring the object distance, or in the amount of reception of the reflection thereof is slight, and which have a spectral transmittance characteristic which matches well with the sensitivity characteristic of an image receiver, such as photographic film or a video image pickup device.
Many existing photographic optical systems are equipped with focus detectors of the TTL (through-the-lens) type. A wide variety of arrangements for the focus detection are known, such as placing the photo sensor either in the camera body or in the photographic lens, and mounting the projector either on the front panel of the camera body, or in the photographic lens.
When photoelectrically measuring the distance the greater the received light, the easier to achieve improved accuracy in distance measurement. In the past, however, improved performance of the projection lens has been achieved, but there has been no known approach for considering the spectral transmittance characteristics of the photographic lens.
In general, the spectral transmittance characteristics of photographic lenses are such as to match the sensitivity characteristics of the image receiving means, such as photographic films, decreasing with increase in the wavelength to considerably small values in transmittance, for light whose wavelength is longer than 700 nm and for infrared light, as shown in FIG. 8.
This is particularly true when the photographic lens is of the zoom type. Because the zoom uses a great number of lens elements, an extreme loss occurs in the infrared region. With such a photographic optical system, therefore, when infrared light of about 850 nm is used in distance measurement, an extreme loss in the amount of light results, lowering the ability of distance measurement.
If such light loss is then avoided by modifying the spectral characteristics of the photographic optical system, so that the range of wavelengths in which the transmittance is high is extended up to the infrared region, an alternative problem arises that for color films, since only the red light increases in intensity, reproducibility of the color balance of the photographic optical system is lost.
In the photographic optical system of the TTL type, with a beam splitter arranged in an intermediate space of the system to reflect a projecting beam toward the object, when the reflected light from the object is used for distance measurement, the spectral characteristic of the reflection surface of a beam splitter has a transmittance curve, for example, shown in FIG. 9. For the light coming from the object and entering through the photographic optical system, the reflection surface of such spectral transmittance curve functions as a sharp cut filter, thus giving no contribution to the color rendering of the image.
Another method of correcting the spectral energy on the image plane is by putting a near infrared cut filter on the image side of the beam splitter. In this arrangement, the spectral characteristic of the near infrared cut filter is not significantly different from that shown in FIG. 9. Thus, no contribution is given to the color rendering of the image.